To understand the Graal legend, you have to understand the Christ (Messiah) formula. The cup is representative of the spilling of blood through the self sacrifice (pain) of life in this plane.
For 99.9% of Christians, this explanation is total heresy.YOU are Christ
(and Antichrist, if the formula is expanded). The whole thing is an allegory
Why doesn't anybody get that?
The nails represent the binding of the spiritual to the physical & the cross is the Universe (which is universally represented by the symbol 'T', 'Tav' or 'Cross'. All of us are metaphysically 'nailed' to the material plane.
This life is merely a bridge to Heaven, wherein sacrifice to gain Grace occurs. YOU are Christ... get it??
Christ isn't one individual, it's ALL individuals.
He never existed as a single individual in a story, but he does exist (look in the mirror). Get it?
This is what happens when you let churches *interpret* scripture... they twist it into a money making scheme.